1. Field of the Invention
The present invention relates to the field of lithographic processing. More particularly, the present invention relates to methods and systems for optimizing lithographic processing, e.g. extreme ultraviolet lithography, e.g. to methods and devices for measuring and/or studying contamination, like carbon contamination, in a lithographic processing system.
2. Description of the Related Technology
Optical lithography nowadays uses wavelengths of 248 nm or 193 nm. With 193 nm immersion lithography integrated circuit (IC) manufacturing is possible down to 45 nm or even down to 32 nm node. However for printing in sub-32 nm half pitch node, this wavelength is probably not satisfactory due to theoretical limitations, unless double patterning is used. Instead of using wavelengths of 193 nm, a more advanced technology has been introduced, also referred to as extreme ultraviolet lithography (EUV lithography), which uses wavelengths of 10 nm to 14 nm, with as typical value 13.5 nm. This technique was in its earlier stages referred to as soft X-ray lithography, more specifically using wavelengths in the range of 2 nm to 50 nm, but then did not yet make use of reduction optics as typically used in optical lithography.
In optical lithography at some wavelengths in the deep ultra violet (DUV) range, the electromagnetic radiation is transmitted by most materials, including glass used for conventional lenses and masks.
At short wavelengths however, e.g. for extreme ultraviolet lithography and soft X-ray lithography, the electromagnetic radiation is absorbed by most materials, including glass used for conventional lenses and masks. Therefore a completely different tool is necessary for performing EUV lithography compared to conventional optical lithography. Instead of using lenses, such an imaging system presently relies on all-reflective optics and therefore is composed of reflective optical elements, also referred to as catoptric elements, for example mirrors. These reflective optical elements, e.g. mirrors preferably are coated with multi-layer structures designed to have a high reflectivity (up to about 70%) at the 13.5 nm wavelength. Furthermore, since air will also absorb EUV light, a vacuum environment is necessary.
Although EUV lithography is considered applicable using wavelengths less than about 32 nm, still a lot of problems need to be overcome to reach a mature technology. Major issues in EUVL are providing a reliable high power source and collector module, obtaining appropriate resist properties, obtaining a defect free mask, having an appropriate reticle protection and obtaining good projection and illuminator optics quality and lifetime. One of the issues relates to contamination of the optics by chemical components, also referred to as “contamination”, which components are usually gaseous components originating from outgassing of the resist. This resist outgassing occurs due to the EUV irradiation of the EUV resist.
Such outgassing affects the reflectivity of the optical elements used, as contamination reduces the reflectivity of the reticle as well as of the imaging optics, and this results in a fast deterioration of the overall imaging quality. Following the international technology roadmap for semiconductors (ITRS) the organic material outgas sing rate for 2 minutes under the lens should be lower than about 5e13 molecules/cm2 per second.
In order to reduce the resist outgas sing rate, metrology tools are necessary which are able to measure the amount of resist outgassing for certain resists. One possibility for screening resist outgassing is also described in a publication of K. R. Dean et al in Proc. of SPIE 6153E, p. 1-9 (2006). An outgassing chamber is built on a synchrotron beam line. Together with the wafers, a Si3N4 witness plate is put in the chamber and exposed to EUV irradiation. This witness plate is then analyzed with electron spectroscopy for chemical analysis (ESCA) to find evidence of the contamination build up. The contaminants are collected in thermal desorption (TD) tubes. The contaminants in these TD tubes are analyzed by gas chromatography/mass spectroscopy (GC/MS) for chemical analysis. In order to obtain information regarding the real contamination on the different components, the witness plate is to be evaluated in a separate characterization system, thus resulting in a tedious and non-efficient task. Transporting the witness plate from the lithographic system to the characterization system may furthermore lead to further contamination of the witness plate resulting in an inaccurate determination of the contamination due to outgas sing.